Zebrafish assays of ciliopathies

Loss of function mutations in CCDC32 cause a congenital syndrome characterized by craniofacial, cardiac and neurodevelopmental anomalies

Despite the wide use of genomics to investigate the molecular basis of rare congenital malformations, a significant fraction of patients remains bereft of diagnosis. As part of our continuous effort to recruit and perform genomic and functional studies on such cohorts, we investigated the genetic and mechanistic cause of disease in two independent consanguineous families affected by overlapping craniofacial, cardiac, laterality and neurodevelopmental anomalies. Using whole exome sequencing, we identified homozygous frameshift CCDC32 variants in three affected individuals. Functional analysis in a zebrafish model revealed that ccdc32 depletion recapitulates the human phenotypes. Because some of the patient phenotypes overlap defects common to ciliopathies, we asked if loss of CCDC32 might contribute to the dysfunction of this organelle. Consistent with this hypothesis, we show that ccdc32 is required for normal cilia formation in zebrafish embryos and mammalian cell culture, arguing that ciliary defects are at least partially involved in the pathomechanism of this disorder.

Long-Pentraxin 3 Affects Primary Cilium in Zebrafish Embryo and Cancer Cells via the FGF System

Primary cilium drives the left-right asymmetry process during embryonic development. Moreover, its dysregulation contributes to cancer progression by affecting various signaling pathways. The fibroblast growth factor (FGF)/FGF receptor (FGFR) system modulates primary cilium length and plays a pivotal role in embryogenesis and tumor growth. Here, we investigated the impact of the natural FGF trap long-pentraxin 3 (PTX3) on the determination of primary cilium extension in zebrafish embryo and cancer cells. The results demonstrate that down modulation of the PTX3 orthologue ptx3b causes the shortening of primary cilium in zebrafish embryo in a FGF-dependent manner, leading to defects in the left-right asymmetry determination. Conversely, PTX3 upregulation causes the elongation of primary cilium in FGF-dependent cancer cells. Previous observations have identified the PTX3-derived small molecule NSC12 as an orally available FGF trap with anticancer effects on FGF-dependent tumors. In keeping with the non-redundant role of the FGF/FGR system in primary cilium length determination, NSC12 induces the elongation of primary cilium in FGF-dependent tumor cells, thus acting as a ciliogenic anticancer molecule in vitro and in vivo. Together, these findings demonstrate the ability of the natural FGF trap PTX3 to exert a modulatory effect on primary cilium in embryonic development and cancer. Moreover, they set the basis for the design of novel ciliogenic drugs with potential implications for the therapy of FGF-dependent tumors.

Human IFT52 mutations uncover a novel role for the protein in microtubule dynamics and centrosome cohesion

Mutations in genes encoding components of the intraflagellar transport (IFT) complexes have previously been associated with a spectrum of diseases collectively termed ciliopathies. Ciliopathies relate to defects in the formation or function of the cilium, a sensory or motile organelle present on the surface of most cell types. IFT52 is a key component of the IFT-B complex and ensures the interaction of the two subcomplexes, IFT-B1 and IFT-B2. Here, we report novel IFT52 biallelic mutations in cases with a short-rib thoracic dysplasia (SRTD) or a congenital anomaly of kidney and urinary tract (CAKUT). Combining in vitro and in vivo studies in zebrafish, we showed that SRTD-associated missense mutation impairs IFT-B complex assembly and IFT-B2 ciliary localization, resulting in decreased cilia length. In comparison, CAKUT-associated missense mutation has a mild pathogenicity, thus explaining the lack of skeletal defects in CAKUT case. In parallel, we demonstrated that the previously reported homozygous nonsense IFT52 mutation associated with Sensenbrenner syndrome [Girisha et al. (2016) A homozygous nonsense variant in IFT52 is associated with a human skeletal ciliopathy. Clin. Genet., 90, 536-539] leads to exon skipping and results in a partially functional protein. Finally, our work uncovered a novel role for IFT52 in microtubule network regulation. We showed that IFT52 interacts and partially co-localized with centrin at the distal end of centrioles where it is involved in its recruitment and/or maintenance. Alteration of this function likely contributes to centriole splitting observed in Ift52-/- cells. Altogether, our findings allow a better comprehensive genotype-phenotype correlation among IFT52-related cases and revealed a novel, extra-ciliary role for IFT52, i.e. disruption may contribute to pathophysiological mechanisms.

Functional characterization of tektin-1 in motile cilia and evidence for TEKT1 as a new candidate gene for motile ciliopathies

A child presenting with Mainzer-Saldino syndrome (MZSDS), characterized by renal, retinal and skeletal involvements, was also diagnosed with lung infections and airway ciliary dyskinesia. These manifestations suggested dysfunction of both primary and motile cilia, respectively. Targeted exome sequencing identified biallelic mutations in WDR19, encoding an IFT-A subunit previously associated with MZSDS-related chondrodysplasia, Jeune asphyxiating thoracic dysplasia and cranioectodermal dysplasia, linked to primary cilia dysfunction, and in TEKT1 which encodes tektin-1 an uncharacterized member of the tektin family, mutations of which may cause ciliary dyskinesia. Tektin-1 localizes at the centrosome in cycling cells, at basal bodies of both primary and motile cilia and to the axoneme of motile cilia in airway cells. The identified mutations impaired these localizations. In addition, airway cells from the affected individual showed severe motility defects without major ultrastructural changes. Knockdown of tekt1 in zebrafish resulted in phenotypes consistent with a function for tektin-1 in ciliary motility, which was confirmed by live imaging. Finally, experiments in the zebrafish also revealed a synergistic effect of tekt1 and wdr19. Altogether, our data show genetic interactions between WDR19 and TEKT1 likely contributing to the overall clinical phenotype observed in the affected individual and provide strong evidence for TEKT1 as a new candidate gene for primary ciliary dyskinesia.

CFAP70 Is a Novel Axoneme-Binding Protein That Localizes at the Base of the Outer Dynein Arm and Regulates Ciliary Motility

In the present study, we characterized CFAP70, a candidate of cilia-related protein in mice. As this protein has a cluster of tetratricopeptide repeat (TPR) domains like many components of the intraflagellar transport (IFT) complex, we investigated the domain functions of particular interest in ciliary targeting and/or localization. RT-PCR and immunohistochemistry of various mouse tissues demonstrated the association of CFAP70 with motile cilia and flagella. A stepwise extraction of proteins from swine tracheal cilia showed that CFAP70 bound tightly to the ciliary axoneme. Fluorescence microscopy of the cultured ependyma expressing fragments of CFAP70 demonstrated that the N-terminus rather than the C-terminus with the TPR domains was more important for the ciliary localization. When CFAP70 was knocked down in cultured mouse ependyma, reductions in cilia beating frequency were observed. Consistent with these observations, a Chlamydomonas mutant lacking the CFAP70 homolog, FAP70, showed defects in outer dynein arm (ODA) activity and a reduction in flagellar motility. Cryo-electron tomography revealed that the N-terminus of FAP70 resided stably at the base of the ODA. These results demonstrated that CFAP70 is a novel regulatory component of the ODA in motile cilia and flagella, and that the N-terminus is important for its ciliary localization.

Putative digenic inheritance of heterozygous RP1L1 and C2orf71 null mutations in syndromic retinal dystrophy

BACKGROUND

Retinitis pigmentosa (RP) is the most common cause of inherited retinal degeneration and can occur in non-syndromic and syndromic forms. Syndromic RP is accompanied by other symptoms such as intellectual disability, hearing loss, or congenital abnormalities. Both forms are known to exhibit complex genetic interactions that can modulate the penetrance and expressivity of the phenotype.

MATERIALS AND METHODS

In an individual with atypical RP, hearing loss, ataxia and cerebellar atrophy, whole exome sequencing was performed. The candidate pathogenic variants were tested by developing an in vivo zebrafish model and assaying for retinal and cerebellar integrity.

RESULTS

Exome sequencing revealed a complex heterozygous protein-truncating mutation in RP1L1, p.[(Lys111Glnfs*27; Gln2373*)], and a heterozygous nonsense mutation in C2orf71, p.(Ser512*). Mutations in both genes have previously been implicated in autosomal recessive non-syndromic RP, raising the possibility of a digenic model in this family. Functional testing in a zebrafish model for two key phenotypes of the affected person showed that the combinatorial suppression of rp1l1 and c2orf71l induced discrete pathology in terms of reduction of eye size with concomitant loss of rhodopsin in the photoreceptors, and disorganization of the cerebellum.

CONCLUSIONS

We propose that the combination of heterozygous loss-of-function mutations in these genes drives syndromic retinal dystrophy, likely through the genetic interaction of at least two loci. Haploinsufficiency at each of these loci is insufficient to induce overt pathology.

Novel NEK8 Mutations Cause Severe Syndromic Renal Cystic Dysplasia through YAP Dysregulation

Ciliopathies are a group of genetic multi-systemic disorders related to dysfunction of the primary cilium, a sensory organelle present at the cell surface that regulates key signaling pathways during development and tissue homeostasis. In order to identify novel genes whose mutations would cause severe developmental ciliopathies, >500 patients/fetuses were analyzed by a targeted high throughput sequencing approach allowing exome sequencing of >1200 ciliary genes. NEK8/NPHP9 mutations were identified in five cases with severe overlapping phenotypes including renal cystic dysplasia/hypodysplasia, situs inversus, cardiopathy with hypertrophic septum and bile duct paucity. These cases highlight a genotype-phenotype correlation, with missense and nonsense mutations associated with hypodysplasia and enlarged cystic organs, respectively. Functional analyses of NEK8 mutations in patient fibroblasts and mIMCD3 cells showed that these mutations differentially affect ciliogenesis, proliferation/apoptosis/DNA damage response, as well as epithelial morphogenesis. Notably, missense mutations exacerbated some of the defects due to NEK8 loss of function, highlighting their likely gain-of-function effect. We also showed that NEK8 missense and loss-of-function mutations differentially affect the regulation of the main Hippo signaling effector, YAP, as well as the expression of its target genes in patient fibroblasts and renal cells. YAP imbalance was also observed in enlarged spheroids of Nek8-invalidated renal epithelial cells grown in 3D culture, as well as in cystic kidneys of Jck mice. Moreover, co-injection of nek8 MO with WT or mutated NEK8-GFP RNA in zebrafish embryos led to shortened dorsally curved body axis, similar to embryos injected with human YAP RNA. Finally, treatment with Verteporfin, an inhibitor of YAP transcriptional activity, partially rescued the 3D spheroid defects of Nek8-invalidated cells and the abnormalities of NEK8-overexpressing zebrafish embryos. Altogether, our study demonstrates that NEK8 human mutations cause major organ developmental defects due to altered ciliogenesis and cell differentiation/proliferation through deregulation of the Hippo pathway.

Genes mutated in ciliopathies encode proteins with various localizations and functions at the primary cilium. Here we report novel NEK8 mutations in patients with renal cystic hypodysplasia and associated ciliopathy defects. NEK8 belongs to a protein complex defining the Inversin compartment of the cilium. It is also a negative regulator of the Hippo signaling pathway that controls organ growth. We report genotype-phenotype correlation in the patients. We functionally demonstrate that the two types of mutations (missense versus nonsense) differentially affect ciliogenesis, cell apoptosis and epithelialisation. We also show that all the mutations lead to dysregulation of the Hippo pathway through nuclear YAP imbalance but that the nature of this imbalance is different according to the type of mutation. We confirm alteration of the Hippo pathway associated with Nek8 mutation in vivo in Jck mice. Remarkably, we show that morphogenesis defects observed in Nek8 knockdown epithelial cells or zebrafish embryos are rescued by Verteporfin, a specific inhibitor of YAP transcriptional activity, demonstrating the causative role of YAP dysregulation in the occurrence of these defects. Altogether, this study links NEK8 mutations to dysregulation of the Hippo pathway and provide molecular clues to understand the variability of the multiorgan defects in the patients.

Genomic approaches for the discovery of genes mutated in inherited retinal degeneration

In view of their high degree of genetic heterogeneity, inherited retinal diseases (IRDs) pose a significant challenge for identifying novel genetic causes. Thus far, more than 200 genes have been found to be mutated in IRDs, which together contain causal variants in >80% of the cases. Accurate genetic diagnostics is particularly important for isolated cases, in which X-linked and de novo autosomal dominant variants are not uncommon. In addition, new gene- or mutation-specific therapies are emerging, underlining the importance of identifying causative mutations in each individual. Sanger sequencing of selected genes followed by cost-effective targeted next-generation sequencing (NGS) can identify defects in known IRD-associated genes in the majority of the cases. Exome NGS in combination with genetic linkage or homozygosity mapping studies can aid the identification of the remaining causal genes. As these are thought to be mutated in <1% of the cases, validation through functional modeling in, for example, zebrafish and/or replication through the genotyping of large patient cohorts is required. In the near future, whole genome NGS in combination with transcriptome NGS may reveal mutations that are currently hidden in the noncoding regions of the human genome.

Next-generation sequencing for research and diagnostics in kidney disease

The advent of next-generation sequencing technologies has enabled genetic nephrology research to move beyond single gene analysis to the simultaneous investigation of hundreds of genes and entire pathways. These new sequencing approaches have been used to identify and characterize causal factors that underlie inherited heterogeneous kidney diseases such as nephronophthisis and congenital anomalies of the kidney and urinary tract. In this Review, we describe the development of next-generation sequencing in basic and clinical research and discuss the implementation of this novel technology in routine patient management. Widespread use of targeted and nontargeted approaches for gene identification in clinical practice will require consistent phenotyping, appropriate disease modelling and collaborative efforts to combine and integrate data analyses. Next-generation sequencing is an exceptionally promising technique that has the potential to improve the management of patients with inherited kidney diseases. However, identifying the molecular mechanisms that lead to renal developmental disorders and ciliopathies is difficult. A major challenge in the near future will be how best to integrate data obtained using next-generation sequencing with personalized medicine, including use of high-throughput disease modelling as a tool to support the clinical diagnosis of kidney diseases.

A mutation in PAK3 with a dual molecular effect deregulates the RAS/MAPK pathway and drives an X-linked syndromic phenotype

Loss-of-function mutations in PAK3 contribute to non-syndromic X-linked intellectual disability (NS-XLID) by affecting dendritic spine density and morphology. Linkage analysis in a three-generation family with affected males showing ID, agenesis of corpus callosum, cerebellar hypoplasia, microcephaly and ichthyosis, revealed a candidate disease locus in Xq21.33q24 encompassing over 280 genes. Subsequent to sequencing all coding exons of the X chromosome, we identified a single novel variant within the linkage region, affecting a conserved codon of PAK3. Biochemical studies showed that, similar to previous NS-XLID-associated lesions, the predicted amino acid substitution (Lys389Asn) abolished the kinase activity of PAK3. In addition, the introduced residue conferred a dominant-negative function to the protein that drives the syndromic phenotype. Using a combination of in vitro and in vivo studies in zebrafish embryos, we show that PAK3(N389) escapes its physiologic degradation and is able to perturb MAPK signaling via an uncontrolled kinase-independent function, which in turn leads to alterations of cerebral and craniofacial structures in vivo. Our data expand the spectrum of phenotypes associated with PAK3 mutations, characterize a novel mechanism resulting in a dual molecular effect of the same mutation with a complex PAK3 functional deregulation and provide evidence for a direct functional impact of aberrant PAK3 function on MAPK signaling.

Acute versus chronic loss of mammalian Azi1/Cep131 results in distinct ciliary phenotypes

Defects in cilium and centrosome function result in a spectrum of clinically-related disorders, known as ciliopathies. However, the complex molecular composition of these structures confounds functional dissection of what any individual gene product is doing under normal and disease conditions. As part of an siRNA screen for genes involved in mammalian ciliogenesis, we and others have identified the conserved centrosomal protein Azi1/Cep131 as required for cilia formation, supporting previous Danio rerio and Drosophila melanogaster mutant studies. Acute loss of Azi1 by knock-down in mouse fibroblasts leads to a robust reduction in ciliogenesis, which we rescue by expressing siRNA-resistant Azi1-GFP. Localisation studies show Azi1 localises to centriolar satellites, and traffics along microtubules becoming enriched around the basal body. Azi1 also localises to the transition zone, a structure important for regulating traffic into the ciliary compartment. To study the requirement of Azi1 during development and tissue homeostasis, Azi1 null mice were generated (Azi1^Gt/Gt). Surprisingly, Azi1^Gt/Gt MEFs have no discernible ciliary phenotype and moreover are resistant to Azi1 siRNA knock-down, demonstrating that a compensation mechanism exists to allow ciliogenesis to proceed despite the lack of Azi1. Cilia throughout Azi1 null mice are functionally normal, as embryonic patterning and adult homeostasis are grossly unaffected. However, in the highly specialised sperm flagella, the loss of Azi1 is not compensated, leading to striking microtubule-based trafficking defects in both the manchette and the flagella, resulting in male infertility. Our analysis of Azi1 knock-down (acute loss) versus gene deletion (chronic loss) suggests that Azi1 plays a conserved, but non-essential trafficking role in ciliogenesis. Importantly, our in vivo analysis reveals Azi1 mediates novel trafficking functions necessary for flagellogenesis. Our study highlights the importance of both acute removal of a protein, in addition to mouse knock-out studies, when functionally characterising candidates for human disease.

Cilia are slender projections from the surface of most mammalian cells and have sensory and sometimes motile functions. They are essential for mammalian development and defects in cilia lead to a group of human diseases, termed ciliopathies, with variable symptoms including embryonic lethality, lung and kidney defects, blindness and infertility. Cilia are complex structures composed of hundreds of components, whose individual functions are poorly understood. We screened for mammalian genes important in building cilia, and identified Azi1/Cep131, a gene previously shown to be required for cilia formation and function in fish and flies. We show that if we acutely reduce levels of Azi1 in mouse cells, fewer cells form cilia, but if we generate cells chronically lacking all Azi1, cilia form normally. In addition, mice without any Azi1 are healthy and viable, confirming normal cilia function. However, in these mice, the highly specialised ciliary structure of the sperm tail does not form, resulting in male infertility. We suggest Azi1 has conserved trafficking roles in both primary cilia and the specialised sperm flagella. Abruptly removing Azi1 results in instability causing the existing cilia network to collapse, whereas chronic deletion of Azi1 allows the system to re-equilibrate, and cilia to form normally.

Ataxia, dementia, and hypogonadotropism caused by disordered ubiquitination

BACKGROUND

The combination of ataxia and hypogonadism was first described more than a century ago, but its genetic basis has remained elusive.

METHODS

We performed whole-exome sequencing in a patient with ataxia and hypogonadotropic hypogonadism, followed by targeted sequencing of candidate genes in similarly affected patients. Neurologic and reproductive endocrine phenotypes were characterized in detail. The effects of sequence variants and the presence of an epistatic interaction were tested in a zebrafish model.

RESULTS

Digenic homozygous mutations in RNF216 and OTUD4, which encode a ubiquitin E3 ligase and a deubiquitinase, respectively, were found in three affected siblings in a consanguineous family. Additional screening identified compound heterozygous truncating mutations in RNF216 in an unrelated patient and single heterozygous deleterious mutations in four other patients. Knockdown of rnf216 or otud4 in zebrafish embryos induced defects in the eye, optic tectum, and cerebellum; combinatorial suppression of both genes exacerbated these phenotypes, which were rescued by nonmutant, but not mutant, human RNF216 or OTUD4 messenger RNA. All patients had progressive ataxia and dementia. Neuronal loss was observed in cerebellar pathways and the hippocampus; surviving hippocampal neurons contained ubiquitin-immunoreactive intranuclear inclusions. Defects were detected at the hypothalamic and pituitary levels of the reproductive endocrine axis.

CONCLUSIONS

The syndrome of hypogonadotropic hypogonadism, ataxia, and dementia can be caused by inactivating mutations in RNF216 or by the combination of mutations in RNF216 and OTUD4. These findings link disordered ubiquitination to neurodegeneration and reproductive dysfunction and highlight the power of whole-exome sequencing in combination with functional studies to unveil genetic interactions that cause disease. (Funded by the National Institutes of Health and others.).

Cftr controls lumen expansion and function of Kupffer's vesicle in zebrafish

Regulated fluid secretion is crucial for the function of most organs. In vertebrates, the chloride channel cystic fibrosis transmembrane conductance regulator (CFTR) is a master regulator of fluid secretion. Although the biophysical properties of CFTR have been well characterized in vitro, little is known about its in vivo role during development. Here, we investigated the function of Cftr during zebrafish development by generating several cftr mutant alleles using TAL effector nucleases. We found that loss of cftr function leads to organ laterality defects. In zebrafish, left-right (LR) asymmetry requires cilia-driven fluid flow within the lumen of Kupffer's vesicle (KV). Using live imaging we found that KV morphogenesis is disrupted in cftr mutants. Loss of Cftr-mediated fluid secretion impairs KV lumen expansion leading to defects in organ laterality. Using bacterial artificial chromosome recombineering, we generated transgenic fish expressing functional Cftr fusion proteins with fluorescent tags under the control of the cftr promoter. The transgenes completely rescued the cftr mutant phenotype. Live imaging of these transgenic lines showed that Cftr is localized to the apical membrane of the epithelial cells in KV during lumen formation. Pharmacological stimulation of Cftr-dependent fluid secretion led to an expansion of the KV lumen. Conversely, inhibition of ion gradient formation impaired KV lumen inflation. Interestingly, cilia formation and motility in KV were not affected, suggesting that fluid secretion and flow are independently controlled in KV. These findings uncover a new role for cftr in KV morphogenesis and function during zebrafish development.

Inferring causality and functional significance of human coding DNA variants

Sequencing technology enables the complete characterization of human genetic variation. Statistical genetics studies identify numerous loci linked to or associated with phenotypes of direct medical interest. The major remaining challenge is to characterize functionally significant alleles that are causally implicated in the genetic basis of human traits. Here, I review three sources of evidence for the functional significance of human DNA variants in protein-coding genes. These include (i) statistical genetics considerations such as co-segregation with the phenotype, allele frequency in unaffected controls and recurrence; (ii) in vitro functional assays and model organism experiments; and (iii) computational methods for predicting the functional effect of amino acid substitutions. In spite of many successes of recent studies, functional characterization of human allelic variants remains problematic.

Zebrafish models in cardiac development and congenital heart birth defects

The zebrafish has become an ideal vertebrate animal system for investigating cardiac development due to its genetic tractability, external fertilization, early optical clarity and ability to survive without a functional cardiovascular system during development. In particular, recent advances in imaging techniques and the creation of zebrafish transgenics now permit the in vivo analysis of the dynamic cellular events that transpire during cardiac morphogenesis. As a result, the combination of these salient features provides detailed insight as to how specific genes may influence cardiac development at the cellular level. In this review, we will highlight how the zebrafish has been utilized to elucidate not only the underlying mechanisms of cardiac development and human congenital heart diseases (CHDs), but also potential pathways that may modulate cardiac regeneration. Thus, we have organized this review based on the major categories of CHDs-structural heart, functional heart, and vascular/great vessel defects, and will conclude with how the zebrafish may be further used to contribute to our understanding of specific human CHDs in the future.